This invention relates to a consumable electrode type d.c. arc we and, especially,to an improved invertor type d.c. arc welder with a consumable welding electrode, which is provided with novel means for preventing occurrence of undesirable spattering.
The inverter type d.c. arc welder, which has been developed for making a welding power source device small in size and weight, is arranged to effect voltage transformation with a lightweighted inverter compsoed of semi-conductor switching elements without use of a heavy input transformer. More specifically, as disclosed in the Japanese patent opening gazette No. S61-216859 for example, in a welder of this kind, its welding d.c. power is produced by once rectifying and smoothing an input a.c. power, then converting it into a high frequency a.c. power by the inverter, reducing its voltage to a suitable level by a small-sized and lightweighted output transformer and again rectifying it to produce a d.c. power for welding. In the cited device, its average output voltage is stabilized by detecting variation of the output voltage and controlling a conduction time of the switching elements of the inverter with a resultant detection signal.
While consumable electrode type d.c. arc welding such as C0.sub.2 welding, MAG welding and MIG welding is effected by supplying the above-mentioned stabilized d.c. power from the welder to a welding load formed with a continuously fed welding electrode rod and a base material to be welded, separating the electrode rod shorted to the base material therefrom to form an arc and, thereafter, similarly repeating shorting and arcing, undesirable spattering may occur if the d.c. power supplied to the welding load becomes excessive at the time of shorting. It has been a general practice for suppressing this spattering to provide a large d.c. reactor in the output side of the welder for preventing sudden increase of the welding current, but it has been a bar to small-sizing and lightweighting of the device.
In order to prevent spattering without a larger reactor, it is considered to detect a shorting period from variation of the output voltage and control the inverter as above-mentioned on the basis thereof to reduce the output voltage during this period. The output voltage of the welder varies originally as shown in FIG. 1(a) and, therefore, it should be easy to distinguish therefrom a shorting period Ts and an arcing period Ta. In practice, however, noises are superimposed thereon to make their transitional points unclear. In other words, the voltage across the output terminals of the welder includes high frequency ripples as shown in FIG. 1(b) since the output terminals and the welding load are mutually connected through long cables of 10 to 20 meters long, for example. Therefore, erroneous detection is liable to occur and complete prevention of spattering is difficult. Although it is possible to reduce influence of the high frequency ripples by using the voltage across the welding load instead of the voltage between the output terminals, it is necessary to change connection of a voltage detector every time the electrode or base material is changed and this operation is not only troublesome but also dangerous since conductors of the detector may break to cause electrical leakage.
Accordingly, an object of this invention is to provide an improved inverter type d.c. arc welder with a consumable electrode, including novel means for effectively preventing spattering without any troublesome operation as above.